Coupled feldspar dissolution and secondary mineral precipitation in batch systems: 6. Labradorite dissolution, calcite growth, and clay precipitation at 60 °C and pH 8.2–8.4

Abstract

We conducted experiments on concurrent labradorite dissolution, calcite precipitation, and clay precipitation in batch reactor systems and tracked reaction processes using multiple isotope tracers. Labradorite was chosen for its role as a major and reactive component in basalt; the experiments thus directly impact our understanding of CO₂ storage in basalt aquifers and enhanced rock weathering. We doped initial solutions with ²⁹Si, ⁴³Ca, and Ca¹³CO₃(s). Experiments were conducted at 60 °C and pH ∼ 8.3 for up to 840 h, with isotope ratios in the experimental aqueous solutions measured using MC-ICP-MS. Unidirectional rates of labradorite dissolution near equilibrium were approximately two orders of magnitude slower than far-from-equilibrium rates reported in the literature. Calcite growth occurred near equilibrium and the rates were limited by the labradorite dissolution rates.

In the steady state phase, the interplay of these three heterogeneous reactions—labradorite dissolution, calcite growth, and clay precipitation—results in a coupled system that approaches a near-equilibrium state. The system does not reach true equilibrium because labradorite continues to dissolve, albeit at a much slower rate near equilibrium. The overall reaction can be approximated as,

Na_0.4Ca_0.6Al_1.6Si_2.4O₈ + 0.6HCO₃^- + 1·.7H₂O + 0.4H⁺ → 0.4Na⁺ + 0.6CaCO_3(s) + 0.5Al₂Si₂O₅(OH)_4(s) + 0.6Al(OH)₄^- + 1.4SiO₂^o_(aq)

The experimental results show that using short-term far-from-equilibrium rate constants would lead to an overestimation of feldspar weathering rates at the Earth’s surface (e.g., basalt weathering and enhanced rock weathering) and CO₂ mineralization in basalt aquifers.